A laser range finding apparatus utilizes pulse transit time for determining a distance of an object within a measurement range is provided with a light deflecting device which enables directional scan of a measurement region for the location of objects in the measurement region.
The pulse transit time method for distance measurement is fundamentally known.
In a laser range finding apparatus utilizing pulse transit time for determining a distance of an object within a measurement range, directional location of objects is included. The device includes a pulsed laser which controllably sends light pulses of predetermined pulse duration into a measurement region; a photoreceiver arrangement which receives the light pulses reflected back from an object located in the measurement region; and, an evaluation circuit which derives a distance signal characteristic for the distance of the object from the pulsed laser from the pulse transit time between the transmission and reception of a light pulse utilizing the speed of light. The improvement herein includes a light deflecting device arranged between the measurement region and the pulsed laser for deflecting sequential light pulses with angular position into the measurement region together with means for simultaneously transmitting to the evaluation circuit an angular position signal representative for its instantaneous angular position. An evaluation circuit derives a location of the object within the measurement range from the distance signal and the angular position signal.
The object of the present invention is to use this pulse transit time method for the determination of the position of objects in spatial regions, for example in connection with the security of driverless transport systems and also the general securing of regions.
The invention thus provides a laser radar by means of which not only the spacing of the object hit by the pulsed light from the apparatus can be determined, but rather also the angle at which the object is arranged relative to some basic direction in space.
The light deflecting device has an angular scanning range greater than 90.degree. , and smaller than 270.degree. , and preferably around 180.degree. . The pulsed light beams define a preferably horizontal scanning plane. The pulse duration of the light pulse is short relative to deflection between the sequential light pulses of the light deflecting device enabling the light deflecting device to be approximated as stationary by the evaluation circuit. angular range of approximately 1.degree. is swept over by the light deflection device in 50 to 150, and in particular in 100 .mu.s. If, on the other hand, a light pulse with short duration is transmitted approximately every 50 .mu.s then this signifies that a light pulse is transmitted approximately every half a degree, or 360 pulses over a total scanned range of 180.degree. . This is fully sufficient for the required angular resolution in the safety region.
The time between two transmitted light pulses of approximately 50 .mu.s is exploited for the tests described further below.
Of particular advantage are embodiments of the light deflecting device. For example, the light deflecting device (15) includes a preferably flat rotary mirror. This mirror is rotable about one of the incident light beams. The axis of rotation extends at an angle between 30.degree. to 60.degree. and preferably 45.degree. to the surface of the rotary mirror. It is preferred that that the rotary mirror be of a disc shape.
The rotary mirror receives transmitted pulses, essentially from above, and radiates those pulses essentially horizontally. A transmitter lens is formed in front of the laser.
The light deflecting device serves the dual purpose of receiving the returned pulse light, and deflecting it to a photoreceiver arrangement. In the preferred embodiment, the transmitted pulse light beam and the received pulse light beam are coaxial to one another. The transmitted pulse light beam is in a central region of the mirror, while the peripheral region of the mirror has the received and returned light beam.
The photoreceiver arrangement includes a receiver lens which concentrates the received light onto a photoreceiver. This lens is arranged so that it can pick up light incident on the peripheral region of the rotary mirror. An interference filter, tuned to the spectrum of light transmitted by the pulsed laser, is arranged at the input of the photoreceiver. The light deflecting device preferably sweeps through 360.degree. and continuously rotates in one direction of rotation. The speed of rotation is between 1,000 to 3,000 rpm and preferably is at 1,500 rpm. To track the rotation, a device is utilized which indicates the instantaneous angular position of the rotary plate. In this way a scanning of a desired spatial region is ensured in a constructionally compact and optically very effective manner, with the scanning angle going up to 360.degree. but normally however only amounting to 180.degree..
A particular advantage in this respect is the concentric construction of the transmitted and received pulsed light beams. In this way, a clean geometrical beam separation is in particular achieved as well as sensitivity in the close range.
The speeds of rotation are particularly advantageous, since in this way, in conjunction with the pulse repetition frequencies that are used, one obtains an adequate angular and temporal resolution.
In connection with the subsequent embodiments the use of a computer is of particular significance. In this way, the diverse self-monitoring functions of the system can in particular be realised.
The further embodiments of the invention ensure a distance resolution of 5 cm/bit which is fully sufficient for the envisaged monitoring purposes, with one bit being defined by one or a half period of the clock frequency. According to this aspect, the evaluation circuit includes a counter with a preferably fixed pre-set clock frequency. The counter starts on transmission of a light pulse, and stops upon the reflection of that light pulse being received. The clock operates with a frequency in the range of 0.5 to 3 gigahertz, and more preferably at 1.5 gigahertz.
The count is taken by two asynchronous individual counters. One counter responds to positive halfways. The other responds to negative halfways. The two counters are added, and the total of the addition utilized to measure distance.
It is, however, of particular advantage that fault monitoring can be carried out by the use of two individual counters connected in parallel. The sum of the individual counts of the counters is compared with twice the count of one of the counters. Error can be indicated by a difference of more than one bit. This comparison can be carried out after each light pulse is sent and received. Alternately, the comparison can be carried out between the end of one scan and the start of the next scan of a particular scanning range.
A further error test is possible in the pause between two scans of the angular scanning range. In this case, the computer delivers controlled counting pulses to the individual counters, checks the results of the count, and transmits a false signal when the results of the count do not correspond with the input number of count pulses.
Furthermore it is advantageous when, the noise level which is superimposed on the useful pulse signal is taken into account, since both the brightness in the monitored rooms and also the degree of reflection of the monitored articles can fluctuate greatly.
A further advantageous embodiment is included in noting the maximum of a received light pulse, and compensating the measured time in accordance with a correction value, which is related to the maximum value of the received pulse. A measurement accuracy of up to 5 cm/bit can in particular be achieved by this further development of the invention.
An inserted light reflecting or scattering test body, the sensitivity of the photoreceiver arrangement can be measured with respect to a predetermined boundary value. Errors in the transmission and reception system of the apparatus can be found.
It possible to check the problem-free functioning of the preferably used avalanche reception diode. Further, a luminescent diode in the path of the transmitted pulsed light beam can be utilized to have the computer check during the sweeping of the luminous diode, whether the signal-to-noise ratio is at least the same as a predetermined boundary value.
The apparatus of the invention is expediently located in a housing which is closed off in the region of the exit of the transmitted pulsed light beam and of the received pulsed light beam by a front disc curved in accordance with the scanning.
In order to be able to automatically recognise contamination of this front disc which is dangerous for the function of the apparatus and to transmit a contamination error signal in the event of excessive contamination, the apparatus can expediently be constructed. With the front disc crossed at a plurality of points along its periphery by beams of light barriers, which emerge from light transmitters. The light transmitters are arranged in the region of one and face of the front disc, and are received by light receivers arranged in the face of the other disc. The light transmitters and receivers are connected to a computer via multiplexers for sequential control. Pulses and evaluation of received pulses enable the computer to transmit an error signal when the received pulses have dropped below a predetermined value.
The front disc extends obliquely from the top downward in the direction of the rotary mirror. This disc is preferably angled at its lower end. This brings about a double passage of the light barrier beam through the front disc for complete sampling of the state of the front disc. The inclined positioning of the main part of the front disc thereby simultaneously serves to reflect away the disc surface reflection.
As a result of contaminations in the form of a liquid film on the front disc, and in particular an oil film, which do not or only insubstantially impair the passage of light can trigger a contamination signal in that the characteristic of such films is exploited that they also form a smooth surface when they are applied to a rough background. As a result of technical safety requirements at least two oil measurement channels should be provided in order to also detect in an electronic evaluation circuit that one of the light transmitters or receivers has failed.
All desired navigation and error signals can be converted in suitable manner and tapped off via an interface.
The special advantage of the laser radar apparatus of the invention lies in the fact that it is secured against any form of system error. This applies both for errors in the optical region or also in the electronic evaluation circuit.